It is known to produce products, such as distillate fuels and lubes from waxy hydrocarbon feedstocks by catalytic hydrocracking, hydroisomerization, catalytic dewaxing, or a combination thereof. One source of waxy hydrocarbon feedstocks in the future will be from Fischer-Tropsch process units wherein a synthesis gas is reacted over a Group VI or VIII metal catalyst, then mildly hydroisomerized and/or mildly hydrocracked over a suitable catalyst to produce a distillate fuel, or refinery feedstock useful for conversion to a distillate fuel. In recently issued U.S. Pat. No. 5,378,348, good yields of distillate fuels with excellent cold flow properties are produced from waxy Fischer-Tropsch products via an improved fixed bed process wherein the waxy Fischer-Tropsch product is separated into 260.degree. C. minus and 260.degree. C. plus fractions and separately hydroisomerized to make middle distillates. The 260.degree. C. minus fraction, e.g., 160.degree. to 260.degree. C. fraction, is hydrotreated in a first step at mild conditions over a suitable catalyst to remove heteroatoms, then hydroisomerized is a second step over a fixed bed of a Group VIII noble metal catalyst, suitably a platinum or palladium catalyst, to yield jet fuel and a light naphtha by-product. The heavier 260.degree. C. plus fraction, on the other hand, is directly hydrocracked to produce a 160.degree. to 370.degree. C. fraction which is useful as a diesel or jet fuel, or as a blending component for diesel or jet fuel. While this process demonstrates the feasibility of producing distillates with improved cold flow properties from waxy hydrocarbons, there remains a need to provide further improvements in the hydroisomerization.
Further, when the Fischer-Tropsch waxy product is used for lubricating oils, it is necessary that substantially all of the wax be removed in order to achieve the desired low temperature properties. Catalytic and solvent dewaxing are the major processes used in the petroleum industry today for removing this wax. Catalytic dewaxing works by selectively cracking the waxy molecules over a zeolite catalyst. Presently available commercial catalytic dewaxing processes utilize a relatively small pore zeolite, such as a ZSM-5 containing catalyst. These zeolite catalysts are sensitive to water in the system which is formed from oxygenates in the feedstream. Consequently, it is preferred that the stream be first subjected to a heteroatom removal step prior to catalytic dewaxing and preferably prior to both hydroisomerization and catalytic dewaxing.
Heteroatoms such as sulfur, nitrogen, and oxygen are known catalyst poisons and their removal from petroleum feedstocks is often referred to as hydrotreating. Typically, catalytic hydroprocessing, which includes hydrotreating, hydroisomerization, and hydrodewaxing, of liquid-phase petroleum feedstocks is carried out in co-current reactors in which both a preheated liquid feedstock and a hydrogen-containing treat gas are introduced to the reactor at a point, or points, above one or more fixed beds of hydroprocessing catalyst. The liquid feedstock, any vaporized hydrocarbons, and hydrogen-containing treat gas, all flow in a downward direction through the catalyst bed(s). The resulting combined vapor phase and liquid phase effluents are normally separated in a series of one or more separator vessels, or drums, downstream of the reactor. Dissolved gases are normally removed from the recovered liquid stream by gas or steam stripping in yet another downstream vessel or vessels, or in a fractionator.
Conventional co-current catalytic hydroprocessing has met with a great deal of commercial success; however, it has limitations. For example, because of hydrogen consumption and treat gas dilution by light reaction products, hydrogen partial pressure decreases between the reactor inlet and outlet. At the same time, any reactions for removing heteroatoms, such as hydrodesulfurization and hydrodenitrogenation that take place results in increased concentrations of H.sub.2 S, NH.sub.3, or oxygenates. These are all known to inhibit the activity and performance of hydroprocessing catalysts through competitive adsorption on the catalyst. Thus, the downstream portion of catalyst in a conventional co-current reactor is often limited in reactivity because of the simultaneous occurrence of multiple negative effects, such as low H.sub.2 partial pressure and the presence of high concentrations of heteroatom components. Further, liquid phase concentrations of the targeted hydrocarbon reactants are also the lowest at the downstream part of the catalyst bed. Also, because kinetic and thermodynamic limitations can be severe, particularly at deep levels of heteroatom removal, higher reaction temperatures, higher treat gas rates, higher reactor pressures, and often higher catalyst volumes are required. Multistage reactor systems with stripping of heteroatom-containing species between reactors and additional injection of fresh hydrogen-containing treat gas are often employed, but they have the disadvantage of being equipment intensive processes.
Another type of hydroprocessing is countercurrent hydroprocessing which has the potential of overcoming many of these limitations, but is presently of very limited commercial use today. U.S. Pat. No. 3,147,210 discloses a two stage process for the hydrofining-hydrogenation of high-boiling aromatic hydrocarbons. The feedstock is first subjected to catalytic hydrofining, preferably in co-current flow with hydrogen, then subjected to hydrogenation over a sulfur-sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen-containing treat gas. U.S. Pat. Nos. 3,767,562 and 3,775,291 disclose a countercurrent process for producing jet fuels, whereas the jet fuel is first hydrodesulfurized in a co-current mode prior to two stage countercurrent hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two stage co-current/countercurrent process for hydrofining and hydrogenating aromatics in a diesel fuel stream.
While the state of the art relating to producing distillate fuels and lubricant products from Fischer-Tropsch waxes has advanced rapidly over the past decade, there is still a substantial need in the art for ever improved efficient processes for achieving same.